Mitochondria have crucial roles in the life and death of mammalian cells, and help to orchestrate host antiviral defences. Here, we show that the ubiquitous human pathogen herpes simplex virus (HSV) induces rapid and complete degradation of host mitochondrial DNA during productive infection of cultured mammalian cells. The depletion of mitochondrial DNA requires the viral UL12 gene, which encodes a conserved nuclease with orthologues in all herpesviruses. We show that an amino-terminally truncated UL12 isoform-UL12.5-localizes to mitochondria and triggers mitochondrial DNA depletion in the absence of other HSV gene products. By contrast, full-length UL12, a nuclear protein, has little or no effect on mitochondrial DNA levels. Our data document that HSV inflicts massive genetic damage to a crucial host organelle and show a novel mechanism of virus-induced shutoff of host functions, which is likely to contribute to the cell death and tissue damage caused by this widespread human pathogen.